The presence of water in small quantities in gaseous hydrocarbons is harmful to industrial processes and the monitoring of moisture content is necessary to assure proper results. The aluminum oxide electrolytic hygrometer is very good for measuring very low levels of moisture in gases and some liquids if the sample is not harmful toward aluminum or aluminum oxide. The P.sub.2 O.sub.5 electrolytic hygrometer is somewhat less sensitive but very useful for many more harmful gas samples encountered in chemical plants. However, some samples are disruptive of the P.sub.2 O.sub.5 hygrometer. HCl in concentration above 1% gives a background reading because the HCl is partially electrolyzed. For such samples that contain very low ranges of moisture, the signal from the P.sub.2 O.sub.5 hygrometer is low and subject to considerable uncertainty. The levels of moisture which are the concern generally are in the range of 2 to 50 parts per million (ppm).
U.S. Pat. No. 3,244,602 discloses the operation of an electrolytic hygrometer in a repetitive batch titration mode to detect low levels of moisture. A gas sample passes through the hygrometer while it is deenergized such that the hygroscopic material absorbs the moisture from the gas sample. Following a purge of the system, the hygrometer is energized and electrolyzes moisture absorbed from the gas sample. The length of the electrolysis is arbitrarily selected to assure that substantially all of the moisture will be electrolyzed. The moisture content of the gas sample can be calculated from the current-time integral (microampere-minutes) used to electrolyze the moisture as described in U.S. Pat. No. 2,830,945 which describes a continuous operation of electrolytic hygrometers.
Electrolytic hygrometers are generally operated in a continuous mode, rather than in the repetitive batch titration mode because of the time required to electrolyze substantially all of the water absorbed from a gas sample. For example, a typical electrolysis time of 20 minutes is indicated in FIG. 3A for the repetitive batch titration mode hygrometer of U.S. Pat. No. 3,244,602. As a result, increases in the moisture level of hydrocarbon streams would go undetected for at least 20 minutes even if other steps in the analysis could be reduced to a minimum.